Very large amounts of benzene and toluene are consumed annually. These materials find use as chemical intermediates, solvents, and in gasoline.
By far the largest proportion of the available benzene and toluene is made in petroleum refining by the so-called (petroleum naphtha) reforming process. This process is so well known that it need not be described in detail here. Briefly, one of the major reactions in catalytic reforming is the isomerization and dehydrogenation of five and six-membered naphthene compounds contained in the naphtha to form a mixture of benzene, toluene, and xylene which materials can be recovered by distillation and solvent extraction as a mixture commonly referred to as "BTX". This mixture can be resolved by distillation to provide merchant benzene, toluene, and mixed xylenes for further use.
The statutory elimination of lead from most gasolines has compelled refiners to rely heavily on hydrocarbon conversion processes that produce gasoline blending stocks having a high octane number even without the inclusion of lead. Two principal processes for accomplishing this are alkylation and reforming, which produce such high octane blending stocks for the lead-free gasolines. Accordingly, refiners who rely heavily on reforming for gasoline production are understandably reluctant to allow the reformate product to be stripped of the high octane aromatics. There results from this situation a decrease in the available supply of benzene and toluene and a concomitant increase in their cost. There is an evident growing need for alternative methods to manufacture benzene and toluene, methods which do not rely on reformate as the principal source.
Catalytic reforming is a process in which hydrocarbon molecules are rearranged, or reformed in the presence of a catalyst. The molecule rearrangement results in an increase in the octane rating of the feedstock. Thus, during reforming low octane hydrocarbons in the gasoline boiling range are converted into high octane components by dehydrogenation of naphthenes and isomerization, dehydrocyclization and hydrocracking of paraffins.
By way of illustration, the significance of those reactions in reforming can be gleaned from a review of the following table from "Catalysis," vol VI, P. H. Emmett (ed). Copyright 1958 by Litton Educational Publishing Company:
______________________________________ Octane Numbers of Pure Hydrocarbons Blending research octane Hydrocarbon number (clear) ______________________________________ Paraffins: n-Butane 113 N-Pentane 62 n-Hexane 19 n-Heptane 0 n-Octane -19 2-Methylhexane 41 2,2-Dimethylpentane 89 2,2,3-Trimethylbutane 113 Naphthenes (cycloparaffins): Methylcyclopentane 107 1.1-Dimethylcyclopentane 96 Cyclohexane 110 Methylcyclohexane 104 Ethylcyclohexane 43 Aromatics: Benzene 99 Toluene 124 1,3-Dimethylbenzene 145 Isopropylbenzene 132 1,3,5-Trimethylbenzene 171 ______________________________________
Naphtha reforming may also be utilized for the production of benzene, toluene, ethylbenzene, and xylene aromatics. A valuable by-product of naphtha reforming is hydrogen, which may be utilized for hydrotreating and upgrading of other hydrocarbon fractions. Generally, the molecular rearrangement of molecular components of a feed, which occurs during reforming, results in only slight, if any, changes in the boiling point of the reformate (the product of reforming), compared to that of the feed. Accordingly, reforming differs from both cracking and alkylation, both refinery processes, each of which does result in changes of boiling range of the product compared to the feed. That is, in cracking, large molecules are cracked into smaller ones; whereas, in alkylation small molecules are rebuilt into larger molecules.
The most important uses of the reforming process are briefly mentioned: the primary use of catalytic reforming may be concisely stated to be an octane upgrader and a route to premium gasoline. Catalytic reforming is the only refining process that is capable of economically making a gasoline component having high clear research octane ratings. The charge to the reformer (straigh-run, thermal, or hydrocracker naphtha) is usually available in large quantities and is of such low quality that most of it would be unsaleable without reforming.
A correlative use of catalytic reforming is in its ability to produce gasolines of acceptable volatility over a wide range of yields, through proper selection of feedstock and/or operating conditions. The refiner is thus able to vary the yield of gasoline very substantially to meet demand fluctuations. For European demand patterns, where gasoline sales are limiting and it is desired to produce as much middle distillate as practicable, the reformer can be operated on a lighter, lower volume of naphtha to minimize gasoline production while maintaining high crude runs.
Hydrogen, although often considered a by-product, is still a valuable output from the reformer. Normally, it is produced in amounts ranging from 300 to 1200 SCF/Bb1, depending on the type of feed stock and reformer operating conditions. Reformer hydrogen is used to remove unwanted contaminants from reformer feed stocks, for hydrodesulfurization of distillates, hydrocracking of heavy fractions, hydrotreating of lubes and various chemical operations. Hydrogen availability and utilization is expected to assume increasing importance as pollution restrictions lead to increasing hydroprocessing in future years.
The importance of reforming is reflected by data which indicates that finished pool gasoline is about 35% reformate in complex refineries, but can run as high as 80% in topping-reforming refineries. As lead is phased out of gasoline, more and more straight run stocks which are now blended directly into gasoline will be reformed. All current commercial reformers use a platinum containing catalyst with a hydrogen recycle stream. Within this broad definition, there are a great number of different process designs. More than 75% of the industry's reforming capacity is classified as semi-regenerative. A semi-regenerative reformer is one which runs until the catalyst is coked and then is shut down and regenerated. The time period between regenerations varies from several months to as long as 11/2 years.
Within the category of semi-regenerative reforming, a further breakdown can be made on the basis of operating pressure. Units with separator pressures of 450 psig or higher are considered high pressure units. Those with pressures of 300 psig or less are called low pressure units. Anything in between is intermediate pressure. Most of the older units are high pressure, while the newer designs are low or intermediate pressure. Lower pressures give better reformate yields at a given octane level.
Another type of reformer is the cyclic variety. A cyclic unit has the reactors manifolded in such a way that any reactor can be taken out of reforming service and regenerated while the other reactors are still reforming. The time period between regenerations for a cyclic reactor varies from 2 to 10 days. All cyclics are low pressure.
A third type of reformer that has recently been commercialized is the continuous unit. In this type of reformer, catalyst is withdrawn from the unit during reforming, regenerated in small batches in separate regeneration facilities and then replaced in the unit. The regeneration period for continuous units is about one month. As in the case for cyclic units, all continuous units are low pressure.
The reformer is run to operate at a given octane for the reformate, under adiabatic conditions. Thus, the unit can be run at low octane severity or at high octane severity. By way of explanation, it is noted that in the semi-regenerative process comprising several manifolded units the reformate from the last of the units will be characterized by the desired octane, while that product of the preceding manifolded units will be of successively lower octane. Because of both thermodynamics and kinetics of reforming, cracking, if it occurs, predominates at the end of the reforming operation particularly in the semi-regenerative process (i.e. in the last of three units) and in the continuous process. Cracking of long chain paraffins of low octane value (which decrease the final octane of the reformate) results in decreased liquid yields. The cracking of such paraffins results in products outside the boiling range of the reformate. It also results in deactivation of the catalyst, e.g. by coking and deposit of carbonaceous matter other than coke on the catalyst, in a way that is not attributable to the other reactions occurring during reforming. In the semi regenerative and continuous reforming processes, cracking would predominate in the last unit.
Prior to about 1950 chromium oxide or molybdenum oxide supported on alumina were used to effect the two functions of a reforming catalyst. The hydrogenation-dehydrogenation function for paraffin olefin conversion during reforming is effected by the metals chromium and molybdenum and more recently platinum, rhenium, admixtures thereof and noble-metal containing trimetallic alloys. Isomerization activity was provided by acidified alumina.
From the commercialization of platinum reforming in the middle 1950's to the late 1960's, there were no significant improvements in reforming catalysts.
In the late 1960's dramatic breakthrough in reforming catalysts occurred. This was the introduction of the platinum-rhenium bimetallic catalysts. These catalysts have greatly improved stability compared to platinum-only catalysts. By way of background, the platinum and platinum bimetallic catalysts were generally supported on carriers.
Recently, the patent literature has started to recognize the use of platinum and zeolite containing catalyst compositions in reforming. That is, the zeolite may replace in whole or in part the function of alumina in prior reforming catalysts. U.S. Pat. No. 4,456,527 and U.S. Pat. No. 4,443,326 describe zeolite L as a component in a composition for catalyzing reforming.
Zeolites include naturally occurring and synthetic zeolites. They exhibit catalytic properties for various types of hydrocarbon conversions. Zeolites are porous crystalline aluminosilicates having definite crystalline structure as determined by X-ray diffraction studies. Such zeolites have pores of uniform size which are uniquely determined by unit structure of the crystal. The zeolites are referred to as "molecular sieves" because interconnecting channel systems created by pores of uniform pore size allow a zeolite to selectively absorb molecules of certain dimensions and shapes.
By way of background, one authority has described the zeolites structurally, as "framework" aluminosilicates which are based on an infinitely extending three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygen atoms. Furthermore, the same authority indicates that zeolites may be represented by the empirical formula EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
In the empirical formula, x is equal to or greater than 2, since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, and n is the valence of the cation designated m. D. Breck, ZEOLITE MOLECULAR SIEVES, John Wiley & Sons, New York p. 5 (1974). In the empirical formula, the ratio of the total of silicon and aluminum atoms to oxygen atoms is 1:2. M was described therein to be sodium, potassium, magnesium, calcium, strontium and/or barium, which complete the electrovalence makeup of the empirical formula.
The prior art describes a variety of synthetic zeolites. These zeolites have come to be designated by letter or other convenient symbols, as illustrated by the zeolite. The silicon/aluminum atomic ratio of a given zeolite is often variable. Moreover, in some zeolites, the upper limit of the silicon/aluminum atomic ratio is unbounded. ZSM-5 is one such example wherein the silicon/aluminum atomic ratio is at least 2.5 and up to infinity. U.S. Pat. No. 3,941,871, reissued as U.S. Pat. No. 29,948, discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added aluminum and exhibiting the X-ray diffraction pattern characteristic of ZSM-5. Various patents describe inclusion of elements other than silicon and aluminum in the preparation of zeolites. Cf. U.S. Pat. No. 3,530,064, U.S. Pat. Nos. 4,208,305 and 4,238,318 describe the preparation of silicates in the presence of iron.
Zeolites may be classified by pore size. ZSM-5 is a member of a class of zeolites sometimes referred to as medium pore zeolites. The pore sizes of medium pore zeolites range from about 5 to about 7 Angstroms.
Another class of zeolites sometimes referred to as large pore zeolites include inter alia naturally occurring faujasite, synthetic zeolites X,L,Y and zeolite beta. These zeolites are characterized by pore sizes greater than those of the medium pore zeolites. The pore sizes of large pore zeolites are greater than about 7 Angstroms. Because of the larger pore sizes these latter zeolites may be less (molecule) shape selective.